Liquid crystals (LCs) exhibit signature structural features over a large hierarchy of length scales ranging from molecular to macroscopic. The covalent chemical structure of the LC molecules (mesogens, ˜10-100 Å)) provides the anisotropic fluid self-assembly termed a liquid crystal (˜1000 Å-1 μm). However, at macroscopic length scales (˜100s of μm-100 inches) an LC sample will typically possess a great many defects, and the LC will exist as a polydomain.
In order to be useful in applications, typically the LC structure must be controlled over macroscopic length scales in at least one dimension, by a process generally termed alignment. In many LC devices, such as lap top computer displays, high quality alignment (that is alignment without defects perceivable when viewing the display) must be achieved in samples on the order of 5 μm in thickness and ˜100 square inches in area.
Alignment is typically obtained by contacting the LC with a pair of solid substrates, at least one of which has been treated to cause a uniform LC structure over large length scales, forming an LC cell. Often such aligning substrates are comprised of solid glass plates, which my also possess other functionality, such as color filter arrays, thin film transistor arrays or other electrodes, with means of applying electric fields to the LC for electro-optic switching. The glass plates are coated with a polymer thin film, this film having been rendered anisotropic by mechanical rubbing. Such a rubbed polymer film alignment layer then provides the desired uniform alignment of the LC.
The LC mesogens are typically organic molecules possessing a rigid core and one or two flexible tails (FIG. 1A). The mesogens have a large aspect ratio (i.e. they are long and thin); the average long axis of the mesogens being termed the molecular
The LC mesogens are typically organic molecules possessing a rigid core and one or two flexible tails (FIG. 1A). The mesogens have a large aspect ratio (i.e. they are long and thin); the average long axis of the mesogens being termed the molecular director. In the LC phase, the molecular directors are oriented along an axis at the length scale of the LC self-assembly, also termed the director. It is the goal of alignment to orient the LC director macroscopically.
Smectic LCs exhibit at least one phase in which the mesogenic molecules self-assemble into a layered structure with quasi long-range positional order in one dimension; along the layer normal. High quality alignment of smectic LCs requires, in addition to uniform macroscopic orientation of the LC director, also obtaining a macroscopic structure wherein the layers are uniform (e.g. flat and parallel) over the entire sample.
LC main-chain polymers comprise mesogenic monomer units linked covalently along the molecular director to form a polymer possessing two or more mesogenic units (FIG. 1B). In such polymers it is possible to heat the sample into an isotropic fluid phase, and on cooling progress through LC phases, at least one of which is a nematic or smectic phase, and finally freeze into an organic glass possessing the structure of the nematic or smectic LC phase from which the freezing occurs. Smectic LC glasses in particular are useful for various photonic applications, such as second order nonlinear optics applications, especially when the glass freezes from a chiral smectic C* (SmC*) ferroelectric LC (FLC) phase, or from a polar smectic C (SmCP) banana phase, or other polar phases whose structures are well known to those familiar with the smectic LC art; however, in most photonic applications these glasses are only useful if alignment of the smectic layers can be achieved. Furthermore, in many such applications, the LC glass should preferably be a thin film oriented between solid substrates, forming an LC cell as described above.